1. Field of the Invention
The present invention relates to a sheet metal outsert-molded gear for use in a video tape recorder, an audio tape recorder, etc.
2. Description of the Related Art
Hereinafter, a structure of a conventional sheet metal outsert-molded gear is described.
FIG. 5 is a plan view of a conventional sheet metal outsert-molded gear 50. FIG. 6 is a cross-sectional view of the conventional sheet metal outsert-molded gear 50 taken along line B—B of FIG. 5. In FIGS. 5 and 6, reference numeral 1 denotes a disk-shaped sheet metal member. Reference numeral 2 denotes a molded resin gear, which is formed by outsert-molding along the outer perimeter of the disk-shaped sheet metal member 1. In use of the sheet metal outsert-molded gear 50, the molded resin gear 2 is engaged with an external gear (not shown), and is rotated by or transmits rotation to the external gear. Reference numeral 3 denotes a bearing section, which is formed by outsert-molding along the inner perimeter of the disk-shaped sheet metal member 1. Reference numerals 4a, 4b, and 4c denote positions of gates formed in a mold, through which resin is injected for forming the molded resin gear 2 by injection molding (hereinafter, simply referred to as “gate positions”). Reference numerals 5a, 5b, and 5c denote welds, which are interfaces between the resin materials injected from the gate positions 4a, 4b, and 4c. In general, a weld occurs in each intermediate region between the gate positions 4a, 4b, and 4c (e.g., half way between each of the gate positions 4a, 4b, and 4c). In the conventional example illustrated in FIG. 5, a gate position(s) corresponding to a gate(s) of the mold which is used for forming the bearing section 3 by injection molding is omitted.
The dashed lines in FIG. 5 denote the portions of the disk-shaped sheet metal member 1 which lie within the bearing section 3 and molded resin gear 2, as shown in FIG. 6.
Variation in the state of the sheet metal outsert-molded gear 50 with the passage of time (i.e., through use of the sheet metal outsert-molded gear 50 with an external gear as described above) is now described.
FIG. 7 is a plan view showing a fatigued state of the conventional sheet metal outsert-molded gear 50 after use. Reference numeral 6 denotes a weld crack, which is caused at the weld 5a of FIG. 5 due to a variation of the resin material of the molded resin gear 2 with the passage of time through use thereof. That is, the resin material is deformed from an initial state when the molded resin gear 2 is first produced.
In the conventional sheet metal outsert-molded gear 50 having the weld crack 6 in such a fatigued state, an interval between gear teeth 7a and 7b (gear pitch 8) is greater than that of the normal state, i.e., the gear pitch 8 is more than the initial gear pitch when the conventional sheet metal outsert-molded gear 50 is first formed. Thus, the molded resin gear 2 cannot be successfully engaged with the external gear, and accordingly, the rotation of the molded resin gear 2 in association with the external gear is not facilitated. As such, the functional operation of the conventional sheet metal outsert-molded gear 50 is not realized with prolonged use. The present invention provides solutions to such problems of the conventional sheet metal outsert-molded gear.